1. Field of the Invention
The present invention relates to electrical switching apparatus and, more particularly, to apparatus, such as circuit breakers and receptacles, for interrupting a power circuit and, more particularly, to such apparatus for interrupting a power circuit including a glowing contact. The invention also relates to a method for interrupting a power circuit including a glowing contact.
2. Background Information
A glowing contact is a high resistance electrical connection, which can form at the interface of a copper wire and an electrical connection device, such as, for example, a screw terminal of a receptacle. The resulting temperature rise at this connection point can melt the wire's insulation and damage the receptacle. It is desirable to be able to detect this condition and interrupt the current before the glowing contact fault progresses to a hazardous condition.
It is very easy to create a high resistance or glowing contact at a receptacle terminal using copper wire. See, for example, Sletbak, J., et al., “Glowing Contact Areas in Loose Copper Wire Connections,” IEEE, 1991, pp. 244–48.
The hazards associated with glowing contacts, including contacts made with all combinations of copper, brass and iron are known. See Yasuaki Hagimoto, “Japanese Reports on Electrical Fire Causes,”     http://members.ozemail. com.au/˜tcforen/japan/index.html, 1996, 12 pp.
Various standards organizations (e.g., NFPA, CPSC, NIST and NRIPS (Japan)) have shown through extensive studies that a glowing contact is a fire hazard. For example, NRIPS conducted a study showing that at the center of the glowing contact, the temperature exceeds 1250° C., which is above the melting temperature of copper and the flash over temperature of many materials.
Glowing contacts may result from loose connections (e.g., improper splicing with twist-on wire connectors, wires twisted together and wrapped with electrical tape, insufficient tightening of terminals). This type of hazard can happen with as little current as about 0.3 A. When a glowing contact is formed, the current during that formation and the subsequent current flowing through the glowing contact is typically normal, since the voltage drop across a glowing contact is typically about 2 VAC. This typically does not affect most downstream electrical appliances. The existence of a glowing contact, therefore, is not reliably detectable by a conventional upstream current protective device (e.g., a conventional circuit breaker or fuse).
In a CPSC sponsored study of 105 electrical fires in ten different cities, the three dominant conclusions for equipment involved in the ignition included: (1) fixed wiring (37%); (2) cords and plugs (26%); and (3) switches, receptacles and outlets (17%). The most prevalent places for these events to occur are in concealed spaces within ceiling, walls and attics (42%). The main area of origin was the bedroom (17.1%). Of all the equipment involved versus the form of heat of ignition, the largest percentage of heat of ignition was due to loose and faulty connections (26%).
Since the advent of arc fault circuit interrupter (AFCI) technology, the possibility of these occurrences has been greatly reduced. The NFPA has shown that from 1994 to 1998, a total of 73,500 fires had occurred, of which 60,900 fires were caused by electrical arcing. The CPSC has concluded from this data that employing AFCI technology could have prevented about 50% to 75% of those fires.
A significant culprit associated with a glowing contact is copper oxide (Cu2O). There are two recognized mechanisms for creating a high resistance copper oxide contact: arcing; and fretting. The arcing mechanism involves, for example, a loose receptacle screw terminal and slight movement of the wire while it is carrying a current. Every time the electrical connection is broken, a single electrical arc discharge can occur.
Each single arc discharge forms a small amount of copper oxide at the terminal-to-copper wire interface. With repeated discharges, the amount of the copper oxide increases over time. Copper oxide has a number of characteristics which, when combined, creates a hazard. First, the interface can be mechanically strong. Hence, once the terminal-to-copper wire connection is made through the copper oxide, the connection may become permanent. Second, copper oxide is a semiconductor that has a very high negative resistance-versus-temperature characteristic between about 180° C. and about 250° C. Over this temperature range, the resistance decreases as much as five orders of magnitude. As the connection heats, the current tends to concentrate into a relatively narrow region, thereby resulting in a very high current density and temperature. For example, a temperature of about 1200° C. to about 1300° C. may result, which temperature is hot enough to melt, for example, a receptacle's plastic housing, but not the copper oxide. Then, as the terminal heats, the wire insulation begins to fail.
During a glowing contact fault in a receptacle, the copper wire reaches a glowing temperature value at which time the wire looks like an electric heater coil. First, the wire's insulation melts at the terminal and, then, slowly progresses away from the terminal toward other wires in the receptacle's outlet box. This can result in either an arcing fault or a ground fault if the bare glowing wire contacts another conductor. Second, the heat resulting from the glowing contact fault flows into the receptacle and causes the plastic housing of the receptacle to melt. As the plastic melts, the receptacle loses its mechanical integrity and, thus, the electrical isolation between conductors is compromised. This may ultimately lead to either a line-to-ground fault or a neutral-to-ground fault. In the event that the upstream protective device (e.g., a circuit breaker) does not respond, then the plastic could ignite.
AFCIs, ground fault circuit interrupters (GFCIs) and conventional thermal-magnetic circuit breakers all have overcurrent protection in which the magnetic and thermal tripping characteristics meet strict standards. Unfortunately, a glowing contact is a localized condition and the resulting generated heat is not detectable by conventional devices. Hence, significant damage may result to both wire insulation and the receptacle.
U.S. patent application Ser. No. 10/192,580, filed Jul. 10, 2002, discloses a receptacle including a line circuit having a first temperature, a neutral circuit having a second temperature, and a load terminal. Separable contacts are adapted to electrically connect the line circuit and the load terminal. An operating mechanism opens the separable contacts in response to a trip signal. A first diode temperature sensor outputs a first signal representative of the first temperature of the line circuit, and a second diode temperature sensor outputs a second signal representative of the second temperature of the neutral circuit. A difference circuit determines the difference between the first and second signals, and a trip circuit provides the trip signal as a function of the difference.
It is known to employ gas detection technology in industry to detect, for example, natural gas leaks in the home, hazardous chemical leaks, and air quality.
Conventional AFCI devices are intelligent devices, which detect most arcing faults. However, such devices do not detect glowing contact faults until after the occurrence of arcing, or the occurrence of shorting to either ground or neutral. If an AFCI device providing, for example, about 30 ma GFCI protection is employed (e.g., an upstream circuit breaker with both arc and ground fault protection), then that device responds to arcing and/or to a ground fault resulting from insulation damage caused by the glowing contact. Hence, the AFCI/GFCI device will eventually trip, in order to de-energize the branch circuit, thereby protecting the wiring and/or receptacle, which was damaged by the glowing contact. Nevertheless, significant damage may still occur due to overheating in the vicinity of the glowing contact.
There exists the need to provide improved protection from glowing contacts for electrical switching devices, such as receptacles and circuit breakers.